The load factor on an AC current source is usually evaluated by measuring the r.m.s. value of the power or of the current which is taken from it by the user appliance, and by comparing the result of the measurement with the maximum value of the corresponding magnitude as specified in the nominal characteristics of the source.
Although that type of measurement is suitable when the source delivers to a purely resistive load, it is not suitable when the load is a "distorting" load, i.e. having a non-linear characteristic (e.g. a load including an input stage constituted by a rectifier bridge feeding a filtering capacitor). In such a case, the peak value of the current is significantly greater than what the peak value would be for an equivalent non-distorting load, i.e. equal to the r.m.s. value multiplied by .sqroot.2, whereas the peak current may be 2 to 5 times greater than the r.m.s. value if the load is a distorting load. Comparing the r.m.s. values therefore leads to false conclusions which cause the real load on the source to be considerably under-estimated.
Such systematic error cannot be detected by the conventional measuring apparatus with which the source and the user appliance loading the source may be provided, and poses serious drawbacks. Not only is the real load factor on the source not known, but if the intrinsic limit of the source is exceeded at certain times, the reliability of the system is affected, and it may behave wrongly, e.g. it may start limiting, it may cut out, or it may switch over to a back-up system.
An object of the present invention is to remedy those drawbacks, and to make it possible to measure the real load factor on a source, the real load factor being defined as being equal to the ratio between the peak value of the current delivered and the limit value of that one of the nominal characteristics of the source which is specified as being limiting.